1. Field of the Invention
This invention pertains to semiconductor radiation detectors. More particularly, this invention pertains to the integration of the outputs from metal-insulator-semiconductor (MIS) radiation detectors used in an array as an imaging device in which the outputs of the individual detectors are obtained by the charge-injection device (CID) technique (see U.S. Pat. No. 4,079,422 and Burke and Michon, "Charge Injection Imaging: Operating Techniques and Performances Characteristics," IEEE Journal of Solid-State Circuits, pp. 121-128, vol. SC-11, No. 1, February 1976).
2. Description of the Prior Art
The charge injection device technique for reading out an image incident on an array of metal-insulator-semiconductor radiation detectors is known in the prior art; see Burke and Michon, id. Computational methods have been developed to process the output from an array of radiation detectors to compensate for differences between the individual detectors in their sensitivity to incident radiation and in their "zero signal" output in the absence of incident radiation (see U.S. Pat. No. 3,949,162; U.S. Pat. No. 3,800,079; and U.S. Pat. No. 3,800,078). Means have also been developed to automatically adjust the bias voltage on each element of an array of transistors which is used to read out detector arrays by the charge injection device technique, so as to compensate for differences in the threshold voltages among the various readout transistors (see U.S. Pat. No. 4,055,836 and U.S. Pat. No. 4,016,550).
In U.S. Pat. No. 4,079,422, Anagnostopoulos has described a technique for reducing certain distortions in the output of arrays of radiation detectors, which are read out by the charge injection device technique, that are caused by over illumination of some elements in the array. Anagnostopoulos recognized that the strong illumination of some elements in a column of detectors, in effect, altered the capacitance between the column conductor used for readout of the particular column of elements and, as a consequence, the strong illumination altered or distorted the voltage change in the column conductor produced by the readout of one element in the column. (See U.S. Pat. No. 4,079,422.) Anagnostopoulos connected an external, precharged capacitor to the column conductor and measured the voltage change across this external capacitor due to charge transfer in the detector element to measure the output of the particular radiation detector. By reducing the change in the column voltage to a small amount, the external capacitor reduced the effect of changes in capacitance in other elements in the column of the array on the output. Thus, Anagnostopoulos's technique compensates at least in part for the distorting effects of strong illumination of one or more detector elements in a column of an array upon the output of the other detector elements in the same column.
None of the compensation techniques described above, however, compensate for or reduce the distortion effects in each radiation detection element caused by the saturation of the detection mechanism within the same element.
In order to increase the output from each of the elements in an array of radiation detectors, and thus to reduce the noise associated with the output of each individual element (as contrasted with "array" noise associated with differences between the elements), the length of time to which the element is exposed to radiation before the output of the element is read can be increased, thus, in effect, integrating within the radiation detector, the intensity of the incident radiation. The integration of the incident radiation within the detector, however, is limited because the amount of charge that can be created by the incident radiation and collected within each element before the collected charge reaches a saturation level.